Studies on the Proteolytic Bacteria of Milk

Abstract

In previous papers of this series (Frazier and Rupp, 1928), the action of proteolytic bacteria of milk on casein and on skim milk has been reported. In order to make the study of proteolysis in milk more complete a study was made of the action of these organisms on milk from which the casein and lactoglobulin had been removed and in which lactalbumin was the only remaining protein. A number of the proteolytic bacteria were found to cause an increase in ammonia and in amino-nitrogen according to the formol titration method in milk, but were unable to split casein in any of the media used. This led to the supposition that lac-talbumin might have been decomposed. In this work the organisms were grown m a milk serum, made by the removal of all casein and lactoglobulin from milk. This milk serum contained lactalbumin as its only protein and small amounts of non-protein nitrogen. No references have been found to the action of bacteria on milk serum, although Franssen (1929) recommends its use in culture media. The only reference to the action of bacteria on lactalbumin is a preliminary report by Supplee (1917) on the action of milk bacteria on some of the nitrogenous constituents of milk. He obtained results which suggested that lactalbumin might have been decomposed. He grew organimsi milk for twenty-four hours at 30°C. and then analyzed for casein, albumin and other fractions. He sometimes found an increase over the control in the albumin fraction and concluded that the increase was due to proteoses from the casein. In a few cases, however,